Organisms and Their Diversity (Taxonomy)
Organisms and their diversity is the taxonomy domain of the basic-school biology exam. Questions ask you to place an unfamiliar organism into the correct group from a written description, to work through a dichotomous key, and to read and write a binomial name correctly. Marks are lost mainly by choosing a group from appearance instead of from cell structure and nutrition.
The naming system
Every species has a two-part Latin name: the genus is capitalised, the species epithet is lower case, and the pair is italicised in print or underlined by hand - Canis lupus. The hierarchy from broadest to narrowest is domain, kingdom, phylum, class, order, family, genus, species. Relatedness increases as you move down the list: two species in one genus are more closely related than two genera in one family.
The main groups
- Bacteria: prokaryotic - no membrane-bound nucleus, no mitochondria or chloroplasts, circular DNA, peptidoglycan cell wall; divide by binary fission.
- Fungi: eukaryotic, chitin cell wall, no chlorophyll, heterotrophic by absorption; moulds, yeasts and mushrooms; some form mycorrhiza with plant roots, and lichens are a fungus living with an alga or cyanobacterium.
- Plants: eukaryotic, cellulose cell wall, photosynthetic, store starch.
- Animals: eukaryotic, no cell wall, heterotrophic by ingestion, usually motile, with nervous tissue.
- Viruses sit outside the kingdoms: no cells, no metabolism of their own, and they reproduce only inside a host cell.
Worked example
An organism is described as multicellular, with chitin cell walls, no chloroplasts, growing on dead leaves and digesting them externally before absorbing the products. Reasoning step by step: chitin walls rule out plants and animals; digestion outside the body followed by absorption rules out animals; the absence of chlorophyll rules out plants and algae; the combination gives kingdom Fungi - a saprotrophic mould such as Rhizopus. Naming the mode of nutrition, not just the shape, is what earns the mark.
Traps that cost marks
Capitalising the species epithet; calling a virus a bacterium; assuming all bacteria cause disease; using "Protista" as a tidy answer when the question expects a named group; treating an immobile fungus as a plant; forgetting that a lichen is two organisms, not one.